1. Field of the Invention
The present invention relates to a technique for reducing the maximal signal amplitude and hence the crest factor of a signal, for example, a multi-carrier signal such as a DMT (discrete multi-tone) signal, and particularly but not exclusively to such a signal generated by a modem.
2. Background Art
The amplitude distribution of a band-limited, over-sampled signal such as a multi-carrier signal, generated by an ADSL (asymmetric digital subscriber line) modem, can be considered as Gaussian with a zero mean and a variance equal to the power of the transmitter signal. The envelope of such multi-carrier signals randomly present, with a small probability, peaks of a very high amplitude. The span (i.e. the peak to peak voltage) of signals driven by the line driver, which is the component dedicated to transmit the power on the line, is important. The power consumption of the line driver is strongly related to the span of the voltage. In order to keep a reasonable level of power consumption, a certain amount of “clipping” of voltage peaks is desired. Such clipping reduces the voltage peaks of the signal.
However, any such clipping generates broadband noise, usually on the whole bandwidth of the signal. This noise causes loss of data: in the transmit direction by suddenly increasing the background noise; and in the receive direction due to the leakage of the noise through the hybrid. This effect in the receive direction is mainly visible in systems utilising echo cancellers.
Consequently, too frequent clipping causes degradation of the overall bit error ratio of the transmission, resulting in data being lost. Thus, whilst a small amount of clipping is allowable and tolerable, the probability of clipping occurring must be small enough to ensure good performance, i.e. no significant data loss.
U.S. Pat. No. 6,038,261 discloses a method for set-up of a signal in multi-carrier modulation, including clipping of the signal amplitude. A feedback loop is utilised to reinject a clipping noise from a previous pulse into a new pulse. The clipping noise is redistributed outside the useful slip of the signal. This method ensures that the signal never exceeds a predetermined threshold, since the output signal is obtained directly from the output of a clipping circuit. However, the use of a feedback loop makes the behaviour of the technique unpredictable when processing a sequence of pulses. Whilst the technique works in a predictable fashion when processing two successive peaks, feeding the re-distributed noise of the first back to the second, with a sequence of peaks operation would be unstable. With a sequence of peaks it would be difficult to predict the contribution made by successive peaks in the feedback loop, and the system would be too unpredictable for reliable operation.
It is an object of the present invention to provide a technique that reduces the maximum signal amplitude of a multi-carrier signal whilst maintaining the integrity of the signal. In particular the invention aims to reduce the maximum signal amplitude whilst maintaining the bit error ratio of the original multi-carrier signal. Consequently the invention reduces the crest factor of the signal.